Most of the antipsychotics are readily but incompletely absorbed, and many undergo significant first-pass metabolism. The oral bioavailability of chlorpromazine and thioridazine is in the range of 25 to 35%, while that of haloperidol, which is less likely to be metabolized, has an oral bioavailability of about 65%.The antipsychotics are highly lipid soluble and are about 95% bound to proteins. Generally they have a much longer clinical duration of action than could be estimated from their plasma half-lives; this is likely due to their sequestration in fat tissue. Depot preparations are more slowly absorbed and longer acting, and thus can be administered par-enterally at intervals up to 3 weeks. The main routes of metabolism are mediated by hepatic oxidative microso-mal enzymes and by glucuronidation. Some metabolites, such as 7-hydroxychlorpromazine, retain measurable activity, but this effect is not considered to be clinically important; an exception to this observation is the major metabolite of thioridazine, which is more potent than the parent drug. Since drug blood concentrations of the less potent antipsychotics are lower after several weeks of treatment at the same dose, it is believed that these compounds may weakly induce their own metabolism. Also, the ability to metabolize and eliminate these drugs has been shown to diminish with age. Typical elimination half-lives vary from 12 to 24 hours.
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